Such an apparatus and process is already known. The user may be, for example, a reactor for chemical vapor deposition (CVD). Such CVD reactors are used in the manufacture of integrated microcircuits and similar electronic components. To meet the increasingly rigorous requirements for such components, substances of very high purity must be used for the chemical vapor deposition process. The substances involved are usually organic metallic and semi-metallic compounds in liquid form.
In addition to the requirement for very high purity, it is also important to ensure precise compliance with the specified gas mixture composition in the CVD reactor, and therefore a consistent quantity of the substance conveyed to the CVD reactor. In order to prevent changes in the composition due to condensation of the evaporated substance in the line to the CVD reactor, it is necessary to place the bubbler as close as possible to the reactor. The bubbler therefore has to have a small volume, which means that it has to be refilled frequently. For this purpose, in the known apparatus the liquid substance is forced out of the reservoir tank into the bubbler with an inert gas.
The quantity of the substance conveyed to the CVD reactor depends on the distance traversed by the carrier gas bubbles in the liquid substance as they rise through the liquid substance. Because of the evaporation of the liquid substance in the bubbler, this distance becomes progressively shorter, and the quantity of evaporated substance in the carrier gas progressively diminishes. In the known apparatus, the resulting variations in concentration are kept as low as possible by providing level sensors for the minimum level and maximum level, respectively.
Level sensors are, however, inherently relatively imprecise, and are not particularly reliable. The use of two level sensors is therefore associated with significant problems. In addition, the liquid substances used are often pyrophorous, and accordingly a discharge of such a substance from the bubbler due to a malfunction of the level sensor for the maximum level would represent a major hazard.